1. Field of the Invention
The present invention relates generally to fuel injection control apparatuses of internal combustion engines, and more particularly, to a fuel injection control apparatus capable of improving its minimum fuel injection quantity.
2. Description of the Related Art
Internal combustion engines have a fuel injection control apparatus that computes the appropriate quantity of fuel according to a particular operational state and drives a fuel injector used to supply the fuel. The fuel injector opens or closes its valve, part of the injector, by utilizing the magnetic force generated by the flow of current through a solenoid, and thus injects the fuel or stops the injection. The quantity of fuel injected is determined primarily by the differential between the pressure of the fuel and the atmospheric pressure of the injector nozzle and the time during which the valve is maintained in the open state and the fuel is injected. To inject the appropriate quantity of fuel, therefore, it is necessary that the appropriate time for maintaining the open state of the injector valve be assigned according to a particular fuel pressure and that the valve be opened or closed rapidly and accurately.
In this case, delay in the response of the current circuit causes the closing operation of the injector valve to be completed with a delay behind the timing in which the fuel injection control apparatus intends to make the injector close the valve. When the driving pulse Ti applied to the injector is long, a departure of the injection quantity from its desired value due to the delay in the closing of the valve can be avoided by preassigning a power distribution time minus the valve-closing delay. When the duration of power distribution to the injector is short, however, setting the power distribution time minus the valve-closing delay leads to the injector valve starting to close before it fully opens; thus, the quantity of fuel requested cannot be injected accurately.
Accordingly, in a known technique (see, for example, Japanese Patent No. 3768723), the dynamic range of fuel control quantities is expanded by variably adjusting the over-excitation period at an early stage of the opening operation of the injector valve to a minimum requirement according to the pressure of the fuel injected from the injector.
In another known technique (see, for example, Japanese Patent No. 3562125), before an injection pulse signal period of the minimum pulse width terminates, the solenoid current of the injector is forcibly reduced to a valve-open state retention current level within a short time to proportionate the injection quantity to the injection pulse width, thus controlling the injection quantity accurately.